1. Field of the Invention
The invention relates to an assembly between a metal piece and a piece made of a ceramic material based on silicon carbide (SiC) and/or on carbon (C).
2. Description of the Related Art
The invention finds a preferred application in the field of aviation, for assembling a piece made of a refractory metal alloy such as an alloy based on nickel (Ni) or on cobalt (Co), and a piece made of a ceramic matrix composite material, i.e. a CMC piece. More particularly, the CMC pieces concerned comprise a matrix based on SiC, on C, or a composite matrix of C and SiC, reinforced with fibers of SiC and/or of C. Said matrix may be a single phase matrix (e.g. entirely made of SiC), or a multiphase matrix (for example it may contain at least one other phase having self-healing properties, as described in document FR 2 732 338).
CMC pieces are used in airplane turbojets to replace mechanical pieces that are the most exposed thermo-mechanically speaking, since CMC pieces retain good mechanical properties at high temperatures, thereby requiring less cooling, and they are generally lighter in weight.
Nevertheless, a problem arises in fastening such pieces of ceramic material to the metal pieces that surround them.
Amongst the assembly techniques used at present, there can be found conventional mechanical assembly of the riveting or bolting type. That type of assembly is often found to be unsuitable for reasons of bulk, of weight, and/or of poor dynamic behavior.
Assembly techniques are also known that use brazing for assembling together two pieces of ceramic material. Nevertheless, those techniques are difficult to use for brazing together a piece made of ceramic material and a metal piece, because of the very different thermo-mechanical and physico-chemical behaviors of ceramic and metal materials. In particular, there is a difficulty of a very great difference in thermal expansion between the pieces in question.
The expansion coefficient of a metal alloy is often two to five times greater than that of a coefficient of the ceramic materials used. Thus, during cooling, after the brazing composition has been melted, the relative shrinkage of the metal piece gives rise to a zone that is in compression and a zone that is in traction, respectively in a zone that is adjacent to the joint between the brazing composition and the ceramic material piece, and in a zone that is adjacent to the joint between the brazing composition and the metal piece. This causes the resulting assembly to bend, giving rise to stresses that can cause one of the pieces to break, generally the ceramic material piece since it is the more fragile, and to poor behavior of the brazed joint because of its localized deformation.
In addition, because of the high reactivity between C or SiC and metals (and more particularly transition metals), it is generally found that fragile chemical compounds of the carbide or silica type are formed between the ceramic and metal pieces. These fragile compounds weaken the assembly.